The present invention relates to a technology for reducing the error rate of a read error that is caused by a spacing interval between a write head and a read head, which are formed on a slider in a magnetic disk drive, and more particularly to a technology for reducing the read error rate of a magnetic disk having a narrow track pitch.
In a magnetic disk drive, a magnetic head is formed on a slider to compose a head/slider assembly. A plurality of servo tracks are concentrically written on a magnetic disk. When the magnetic disk rotates while the head/slider assembly is positioned over a specified servo track, a data read/data write operation is circumferentially performed on the associated data track. As shown in FIG. 10(a), the head/slider assembly 3, which is supported by a head support mechanism, move over the circumference whose radius is equivalent to the distance to a rotation center 5 of the head support mechanism. Therefore, when the magnetic head is positioned over a servo track of the magnetic disk 1, an angle is generated between a line 15 (hereinafter referred to as the pivot center line) that joins the rotation center 5 of the head support mechanism to the center of a short rim of the head/slide assembly 3 and a data track tangent line 9, 13 at the center position of the head/slider assembly. This angle is generally called a yaw angle, which varies in the direction of the servo track radius in accordance with the position.
When the head/slider assembly 3 is positioned over an inner data track 7 on the magnetic disk 1, yaw angle θ1 is formed between the tangent line 9 of a servo track 7 and the pivot center line 15. When the head/slider assembly 3 is positioned over an outer servo track 11, yaw angle θ2 is formed between the tangent line 13 of the servo track 11 and the pivot center line 15. As is obvious from FIG. 10(A), the yaw angle varies with the radial position of the head/slider assembly 3 over the magnetic disk.
As shown in FIG. 10(B), an MR or GMR read head 17 and an inductive write head 19 are formed on the same slider. The read head 17 and write head 19 are formed on a slider surface facing the magnetic disk so that the pivot center line 15 passes through the center between a write gap and read gap. A predetermined head spacing interval L is provided between the read head 17 and write head 19. This spacing interval is in the direction of the pivot center line 15.
As regards the head spacing interval L between the write head 19 and read head 17, a predefined distance, which varies in the direction of the magnetic disk radius in accordance with a yaw angle, is generated. This distance is hereinafter referred to as the dual head offset (hereinafter abbreviated to the DHO). The DHO is a physical distance that is determined by a yaw angle and head spacing interval L. In the magnetic disk drive, the gap between the read head 17 and write head 19 is recognized in terms of the calculated number of servo tracks and used for magnetic head positioning control. Therefore, the DHO may be expressed in terms of the calculated number of tracks. In the present specification, the DHO value representing a physical distance is referred to as a DHO [distance] whereas the DHO value calculated as the number of servo tracks is referred to as a DHO [calculation].
When a data write is performed in the magnetic disk drive, the read head 17 reads servo data, which is written on the magnetic disk, to determine the position of the read head 17, and then the write head 19 writes user data. The read head 17 is used for positioning the head/slider assembly 3. The write head 19 whose distance to the read head 17 is the head spacing interval L is used for a write. Therefore, the actual user data write position in the direction of the magnetic disk radius is offset by the DHO [distance] from a target position, which is the position of a servo track used for positioning. Therefore, when a read is to be performed, the read head 17 needs to be placed at a position that is offset from the target position by the DHO [distance]. The DHO [distance] varies with the yaw angle, that is, the slider position relative to the magnetic disk. To properly place the read head at a written position, therefore, it is necessary that the magnetic disk drive accurately recognize the DHO [distance].
A technology disclosed by Patent Document 1 (Japanese Patent Laid-open No. 2000-322848) writes an off-track measurement pattern, which is shown in FIG. 6(b) of that document, into all servo sectors of tracks to enhance the on-track accuracy prevailing when a composite head performs a read. A technology disclosed by Patent Document 2 (Japanese Patent Laid-open No. 11-191274) performs a track following operation by providing such an offset as to position a write head at the center of a track.